This revised Program Project renewal application builds on the collaborative relationships and scientific accomplishments of the past funding period in order to continue to advance our understanding of the role of antigen-specific and cross-reactive (XR) CD8+ T cells in the induction and maintenance of memory responses. Project investigators will use the Epstein Barr virus (EBV) model of infection with unique clinical cohorts to: 1) characterize the lineage relationship between effector and memory CD8+ T cell responses in humans and the factors that influence the evolution of antigen-specific CD8+ T cell responses into the memory CD8+ T cell repertoire; 2) characterize how cross-reactivity influences the evolution of the antigen specific CD8+ T cell repertoire and understand the structural and functional properties of the T cell receptor that contribute to the recognition of antigen-specific or XR ligands; 3) correlate antigen-specific and XR CD8+ T cell frequencies and functional properties with control of viral replication, evolution of viral sequences, and manifestations of disease. Project 1 will use TCR CDR3? and sequencing and genome wide microarray analyses to characterize the lineage relationship between effector and memory CD8+ T cell responses and the factors that influence evolution of EBV-specific CD8+ T cell responses into the memory CD8+ T cell repertoire. Project 1 will also use novel deep sequencing methods to analyze EBV sequence diversity in the oral and peripheral blood compartments and will examine the potential role of EBV epitope specific CD8+ T cell responses in EBV sequence evolution. Project 2 will further characterize the human XR CD8 T cell repertoire and determine how cross-reactivity impacts memory CD8+ T cell selection and function and correlates with disease outcomes. Three core facilities will facilitate the work of Project Investigators. Core A (Clinical and Administrative) will provide scientific leadership; manage fiscal affairs; collect, process, and disburse clinical specimens from research subjects; and develop and maintain a comprehensive clinical and laboratory database. The Tetramer Core (Core B) will provide MHCI-peptide tetramers to all Project Investigators. Core C (Flow Cytometry) provides access to state-of-the-art facilities for FACS-based cell sorting and analyses of virus-specific or XR CD8+ T cells. The proposed studies will improve our understanding of the role of antigen-specific and XR CD8+ T cells in the induction and long-term maintenance of memory CD8+ T cell responses. These studies will also help us to understand variability in human responses to viral infections and how antigen-specific or XR CD8+ T cells contribute to either viral control or disease.